Color printing by an inline laser printer is achieved by scanning a digitized image onto a photoconductor using lasers. Such a printing process is known as electrophotographic printing. The lasers generate beams of laser energy which are pulsed according to the digitized data to be imaged on the photoconductor. The photoconductor typically comprises a drum or a belt coated with a photoconductive material capable of retaining localized electrical charges. Each localized area capable of receiving a charge corresponds to a pixel. Each pixel is charged to a base electrical charge, and then is either exposed or not exposed by the laser, as dictated by the digital data used to pulse the laser. Exposing a pixel corresponds to electrically altering (typically discharging) the localized area from the base electrical charge to a different electrical charge. One charge will attract toner, and the other charge will not. In this manner, toner is selectively transferred to the photoconductor. In most electrophotographic printing processes, the exposed (electrically discharged) pixels attract toner onto the photoconductor. This process is known as discharge area development (DAD). However, in some electrophotographic printing processes the toner is attracted to the un-discharged (i.e., charged) area on the photoconductor. This latter type of electrophotographic printing is known as charge-area-development (CAD). For purposes of discussion, it will be assumed that DAD is used, although the present invention is not limited to DAD.
Once the photoconductor has had the desired toner transferred to it, the toner is then transferred to the finished product medium. This transfer can either be direct or it can be indirect using an intermediate transfer device. The finished product medium typically comprises a sheet of paper, normally white, but can also comprise a transparency or a colored sheet of paper. After the toner is transferred to the finished product medium, it is processed to fix the toner to the medium. This last step is normally accomplished by thermally heating the toner to fuse it to the medium, or applying pressure to the toner on the medium.
There are a variety of known methods for selectively attracting toner to a photoconductor. Generally, each toner has a known electrical potential affinity. Selected areas of the photoconductor are exposed from a base potential to the potential for the selected toner, and then the photoconductor is exposed to the toner so that the toner is attracted to the selectively exposed areas. This latter step is known as developing the photoconductor. In some processes, after the photoconductor is developed by a first toner, the photoconductor is then recharged to the base potential and subsequently exposed and developed by a second toner. In other processes, the photoconductor is not recharged to the base potential after being exposed and developed by a selected toner. In yet another process, the photoconductor is exposed and developed by a plurality of toners, then recharged, and then exposed and developed by another toner. In certain processes, individual photoconductors are individually developed with a dedicated color, and then the toner is transferred from the various photoconductors to a transfer medium which then transfers the toner to the finished product medium. The selection of the charge-expose-develop process depends on a number of variables, such as the type of toner used and the ultimate quality of the image desired. The quality of the final image on the medium is typically associated with complexity and cost of the printer, such that higher quality electrophotographic printers which produce higher quality images are more complex, and concomitantly more expensive.
Image data for a laser printer, including color laser printers, is digital data which is stored in computer memory. The data is stored in a matrix or "raster" which identifies the location and color of each pixel which comprises the overall image. The raster image data can be obtained by scanning an original analog document and digitizing the image into raster date, or by reading an already digitized image file. The former method is more common to photocopiers, while the latter method is more common to printing computer files using an in-line printer. Accordingly, the technology to which the invention described below is applicable to either photocopiers or in-line printers. Recent technology has removed this distinction, such that a single printing apparatus can be used either as a copier or as a printer for computer files. These apparatus have been known as "mopiers", a term indicating the ability to act as a photocopier or an in-line printer. In either event, the image to be printed onto tangible media is stored as a raster image file. The raster image data is then used to pulse the beam of a laser in the manner described above so that the image can be reproduced by the electrophotographic printing apparatus. Accordingly, the expression "printer" should not be considered as limiting to a device for printing a file from a computer, but should also include a photocopier capable of printing a digitized image of an original document. "Original documents" include not already digitized documents such as text and image files, but photographs and other images, including hybrid text-image documents, which are scanned and digitized into raster data.
The raster image data file is essentially organized into a two dimensional matrix. The image is digitized into a number of lines. Each line comprises a number of discrete dots or pixels across the line. Each pixel is assigned a binary value relating information pertaining to its color and potentially other attributes, such as brightness. The combination of lines and pixels makes up the resultant image. The digital image is stored in computer readable memory as a raster image. That is, the image is cataloged by line, and each line is cataloged by each pixel in the line. A computer processor reads the raster image data line by line, and actuates the laser to selectively expose a pixel based on the presence or absence of coloration, and the type of coloration for the pixel. Typical pixel densities for high quality images are in the range of 300 to 1200 pixels per inch, in each direction.
The system for transferring the digital raster data to the photoconductor via a laser or lasers is known as the image scanning process or the scanning process. The scanning process is accomplished by a scanning portion or scanning section of the electrophotographic printer. The process of attracting toner to the photoconductor is known as the image transfer process or the transfer process, and is also known as the development process. The transfer process is accomplished by the transfer portion of the printer, also known as the developer section or develop portion of the electrophotographic printer. Image quality is dependent on both of these processes. Image quality is thus dependent on both the scanning portion of the printer, which transfers the raster data to the photoconductor, as well as the developer section portion of the printer, which manages the transfer of the toner to the photoconductor. The present invention is directed to the scanning process and the scanning section of the electrophotographic printer.
The typical inline color laser printer utilizes a plurality (typically 4) laser scanners to generate a latent electrostatic image for each color plane to be printed. The four color planes typically printed, and which are generally considered as necessary to generate a relatively complete palate of colors, are yellow, magenta, cyan and black. That is, the color printer is typically provided with toners in each of these four colors. These colors will be known herein as the "base colors". Preferably, the printer should have the capability of printing one base color on top of another on the same pixel, so as to generate a fuller palate of finished colors. More preferably, the printer should have the capability of depositing controllably varying amounts of toner on a pixel so as to further expand the palate of available colors.
In the scanning process, a laser is scanned from one edge of the photoconductor to the opposing edge and is selectively actuated or not actuated on a pixel-by-pixel basis to scan a line of the image onto the photoconductor. The photoconductor advances and the next line of the image is scanned by the laser onto the photoconductor. The photoconductor can be advanced discretely or in a continuous mode. In a multiple laser printer, more than one laser can be actuated simultaneously so as to more quickly generate the complete image onto the photoconductor. The side-to-side scanning of each laser is traditionally accomplished using a dedicated multi-sided or faceted rotating mirror. Such a mirror will be known herein as a "polygon" due to the polygonal shape of the mirror. The reflective surface of the mirrors are typically ground and polished aluminum. The laser beam impinges on one facet of the mirror and is reflected to a secondary or deflector mirror, which directs the laser beam to a unique, relative lineal position on the light sensitive surface of the photoconductor. By "relative", it is understood that the photoconductor moves with respect to the linear position, but the position remains fixed in space. As the polygonal mirror rotates, the angle of incidence, and hence the angle of reflection, of the laser beam will vary. This causes the laser beam to be scanned across the photoconductor at its unique relative lineal position from a first edge to a second edge of the photoconductor. As the mirror rotates to an edge of the polygon between facets, the laser is essentially reset to the first edge of the photoconductor to begin scanning a new line onto the advancing photoconductor. These mirrors tend to rotate at very high speeds--typically in excess of 20,000 rpm.
The quality of an image generated with an inline laser printer is usually directly associated with the generation of moire patterns. Moire patterns are undesirable distortions in the image which are the result of a pixel being generated in the photoconductor in a non-representative fashion as compared to the data in the raster image. Moire patterns can be caused by a number of different things, some of which are attributable to the scanning process.
Ideally, the laser impinges on the photoconductor to generate a round pixel. Normally, the laser impinges on the photoconductor at a normal or perpendicular angle at the center of the photoconductor between the two side-to-side edges when the laser is striking the middle of the mirror facet. In this instance, the ideal "round" pixel is generated by the laser. However, as the laser scans across the photoconductor, it will tend to project an elliptical or oval shaped image on the photoconductor due to the changed angle of incidence of the laser on the photoconductor. This distortion is undesirable, and is classified as a moire pattern. This distortion can be corrected to some degree using lenses (commonly known as "f-.theta. lenses" because they focus the beam with respect to the angle .theta. of the rotating polygonal mirror) to focus the laser beam, i.e., to bring the beam back to an essentially normal or perpendicular angel of incidence on the photoconductor. Since each laser needs these corrective lenses, the complexity and cost of the printer is affected.
Another source of moire patters associated with the scanning process is the relative angle error from facet to facet within a single polygonal mirror. Ideally, each facet of the mirror should reflect the laser to the same point on the photoconductor for the same relative point on each mirror. However, due to manufacturing variances or grinding errors between mirror facets, as well as degenerative errors introduced by wear in the bearings supporting the rotating mirror, this is not always so. This results in beam deflection which is typically perpendicular to the side-to-side scan direction. This error can also be corrected to some degree by the use of alignment or centering lenses (typically, cylindrical lenses) to redirect the laser beam to the same relative point on the photoconductor regardless of which facet reflected the beam.
For color printing, it is important to assure the registration of the different colors. That is, each laser should be aligned with respect to the other lasers such that a given pixel in the raster image is associated with a single common point on the photoconductor, regardless of which laser is used to identify the point. A registration which is "off" will result in a blurry image, or an image with colors not representative of the raster image. Registration is thus dependent on aligning all of the lasers in a laser printer. Each laser and its associated components (i.e., rotating mirror, optical elements, and deflector mirror) are typically mounted in a precision housing to keep the components in relative fixed position with respect to one another. The housings are typically castings which are then machined to achieve the desired precision. Assuring registration of the lasers requires aligning the four housings within the printer itself. As environmental conditions within the printer change (e.g., temperature), this alignment can change.
FIG. 1 depicts a schematic side elevation diagram of a prior art four laser color printer "A". The printer "A" comprises a scanning section "B" and a photoconductor section "C". The photoconductor section shown here comprises a rotating belt 5 which supports a photoconductive material. Four developing stations, 6, 7, 8 and 9, are located proximate to the belt 5 and affix toner to the photoconductor in response to selective exposure of the photoconductive material by the laser beams at points "D", "E", "F" and "G" along the belt. For exemplary purposes only, developing station 6 can be the yellow developer, station 7 can be the magenta developer, station 8 can be the cyan developer, and station 9 can be the black developer.
The scanning section "B" in FIG. 1 comprises four scanning laser stations, 11, 12, 13 and 14. Each scanning station comprises a laser 15, a rotating mirror 16, a motor 17 for driving the mirror 16, a laser beam focusing lens 18, an aligning lens 19, a deflector mirror 21 for deflecting the laser beam onto the photoconductor belt 5, and a housing 22 for holding the aforementioned components.
Since only partial alignment of the laser beams with respect to one another can be achieve by aligning the housings which contain the scanning assemblies, in-line color printers are typically also provided with color plane sensors to sense color plane alignment. Sensors are provided to detect shifts in color planes in both the side-to-side scanning direction (the "scan" direction), as well as in the direction of advance of the photoconductor (i.e., the "process" direction). The sensors can provide a feedback to the scanning system and corrections can be made to reposition the laser beam using various known electrical and mechanical methods.
The space required within a printer unit for a plurality of scanning assembly housings tends to reduce the focal length which can be achieved with each laser (i.e., the distance from the focusing lens to the photoconductor surface). Generally, shorter focal lengths require higher quality optics to focus the beam over the shorter distances. Obtaining greater focal lengths with multiple scanning assemblies would require increasing the size of the printer. Since many printers are chosen for desk-top use, a large printer is undesirable.
Each rotating mirror assembly is driven by its own dedicated motor. The power consumption for each mirror driving motor is typically about 20 watts. Thus, for a four-laser printer, the mirror drives alone consume about 80 watts. This requires a larger power supply, generates a fair amount of heat, and generally adds cost and complexity to the overall printer design.
What is needed then is a color printer which reduces the complexity of the scanning section and which also increases the accuracy of the reproduction of the raster image onto the photoconductor.